1. Field of the Invention
This invention relates generally to transferring loads between adjacent cast-in-place slabs and more particularly to a system for transferring, across a joint between a first slab and a second slab, a load applied to either slab.
2. Related Art
Referring to FIG. 1, a concrete floor 100 is typically made up of a series of individual blocks or slabs 102-1 through 102-6 (collectively 102), as shown in FIG. 1. The same is true for sidewalks, driveways, roads, and the like. Blocks 102 provide several advantages including relief of internal stress due to drying shrinkage and thermal movement. Adjacent blocks 102 meet each other at joints, such as joints 104-1 through 104-7 (collectively 104). Joints 104 are typically spaced so that each block 102 has enough strength to overcome internal stresses that would otherwise cause random stress relief cracks. In practice, blocks 102 should be allowed to move individually but should also be able to transfer loads from one block to another block. Transferring loads between blocks 102 is usually accomplished using smooth steel rods, also referred to as dowels, embedded in the two blocks 102 defining the joint 104. For instance, FIG. 2 is a side view of dowel 200 between slabs 102-4 and 102-5. FIG. 3 is a cross-sectional plan view along a section a portion of which is depicted by sectional arrow 3xe2x80x943 in FIG. 2. FIG. 3 shows several dowels 200 spanning joints 104 between slabs 102. Typically, a dowel or bar 200 is approximately 14 to 24 inches long, has either a circular or square cross-sectional shape, and a thickness of approximately 0.5-2 inches. Such circular or square dowels are capable of transferring loads between adjacent slabs 102, but have several shortcomings.
U.S. Pat. Nos, 5,005,331, 5,216,862, and 5,487,249 issued to Shaw et al., which are incorporated herein by reference, disclose tubular dowel receiving sheaths for use with dowel bars having a circular cross-section.
If circular or square dowels, are misaligned (i.e., not positioned perpendicular to joint 104), they can undesirably lock the joint together causing unwanted stresses that could lead to slab failure in the form of cracking. Misaligned dowels 200 are illustrated in FIG. 4. Such misaligned dowels can restrict movement in the directions indicated arrows 400-1 and 400-2.
Another shortcoming of square and round dowels is that they typically allow slabs 102 to move only along the longitudinal axis of the dowel. As shown in FIG. 5, movement in the direction parallel to the dowels 200, as depicted by double-headed arrow 500 is allowed, while movement in other directions, such as the directions indicated arrows 502-1 and 502-2 and the directions which could be referred to as xe2x80x9cinto the pagexe2x80x9d and xe2x80x9cout from the pagexe2x80x9d is restrained. Such restraint of movement in directions other than parallel to the longitudinal axes of dowels 200 could result in slab failure in the form of cracking.
U.S. Pat. No. 4,733,513 (""513 patent) issued to Shrader et al., which is incorporated herein by reference, discloses a dowel bar having a rectangular cross-section and resilient facings attached to the sides of the bar. As disclosed in column 5, at lines 47-49 of the ""513 patent, such bars, when used for typical concrete paving slabs, would have a cross-section on the order of xc2xd to 2-inch square and a length on the order of 2 to 4 feet.
Referring to FIGS. 6 and 7, yet another shortcoming of prior art dowel bars results from the fact that, under a load, only the first 3-4 inches of each dowel bar is typically used for transferring the load. This creates very high loadings per square inch at the edge of slab 102-2, which can result in failure 600 of the concrete below dowel 200, as shown from the side in FIG. 6, and as shown in FIG. 7 along sectional view arrows 7xe2x80x947 in FIG. 6. Such a failure could also occur above dowel 200.
Accordingly, there is a need in the prior art for an improved system that will provide both: (1) increased relative movement between slabs in a direction parallel to the longitudinal axis of the joint; and (2) reduced loadings per square inch close to the joint, while transferring loads between adjacent cast-in-place slabs.
A load plate for transferring loads between a first cast-in-place slab and a second cast-in-place slab separated by a joint. The load plate comprising a substantially tapered end having substantially planar upper and lower surfaces adapted to protrude into and engage the first slab, and the load plate being adapted to transfer between the first and second slabs a load directed substantially perpendicular to the intended upper surface of the first slab. The substantially tapered end could have a largest width, measured parallel to the longitudinal axis of the joint, substantially no less than twice the depth to which the substantially tapered end protrudes into one of the slabs. The height of the load plate, measured perpendicular to the upper surface of the first slab, could be substantially less than one-eighth of the largest width of the substantially tapered end.
A blockout sheath embedded within the first slab could also be included. The block out sheath could have a substantially planar top surface and a substantially planar bottom surface substantially parallel to the upper surface of the first slab. The top and bottom surfaces of the blockout sheath could each have a width, measured parallel to an intersection between the joint surface and the upper surface of the first slab, that substantially decreases away from the joint surface. The width of the blockout sheath could be substantially greater than the width of the substantially tapered end at each corresponding depth along the substantially tapered end and the blockout sheath, such that the substantially tapered end could move within the sheath in a direction parallel to the intersection between the upper surface of the first slab and the joint surface. The blockout sheath could include a plurality of deformable centering fins or other means for initially centering the substantially tapered end of the load plate within the width of the sheath. The largest width of the substantially tapered end of the load plate could be substantially no less than twice the depth of the substantially tapered end. The height of the load plate could be substantially less than one-eighth the largest width of the substantially tapered end of the load plate.
This invention also comprises a load plate kit having component parts capable of being assembled during creation of a joint between first and second cast-in-place slabs including: a mounting plate adapted to be attached to the edge form; a blockout sheath adapted to be attached to the mounting plate; and a load plate such that the load plate and blockout sheath are adapted to transfer a load between the first and second slabs.
This invention also comprises a method of installing a load plate for transferring loads between a first cast-in-place slab and a second cast-in-place slab, including the steps of: placing an edge form on the ground; attaching a substantially tapered blockout sheath to the edge form; removing the edge form from the first slab, with the blockout sheath remaining within the first slab; inserting a substantially tapered end of a load plate into the substantially tapered blockout sheath, a remaining portion of the load plate protruding into a space to be occupied by the second slab; pouring cast-in-place material into the space to be occupied by the second slab; and allowing the second slab to harden.